Alternative Master/Starter Relay Configuration

[kcameron]

Three years ago I mentioned on the VAF forum that I was working on an alternative method of wiring the master and starter relays. I actually built it more than two years ago and have been flying it on my RV-4 for almost that long. I just recently discovered a decent (and free) CAD program to draw it. I had promised to share it with the group so here it is.

The goals of deviating from the standard method (having the starter relay connected to the master relay output) and other methods I've seen are:

1. Reduction of the resistance from battery to starter.
   My method has just one contactor in the starter motor circuit.
2. Eliminating the need for the pilot to operate any additional switch to power-down the avionics during an engine start.
   My circuit uses a single 3-position toggle switch for "Off"/"Start"/"Master".
3. Avoiding the need for a separate avionics master switch.
4. Avoiding any single point of failure that could cause the starter to be energized uncontrollably.
   No single stuck relay can cause the starter to engage. If the start button gets stuck in the on position, I can work around the problem by toggling the main switch into the "Off" or "Master" positions.
5. Avoiding excessive current through the fairly small starter pushbutton switch on the control stick.
   I added an extra relay to buffer the relatively high current needed by the main starter contactor.
6. Providing for an ammeter that measures total battery current except for the starter.
   I like being able to get an instant reading of the health of the system by glancing at the ammeter.
7. Providing for an ammeter that measures battery current even when the plane is powered down.
   Useful when charging via the cigarette lighter plug.
8. Avoiding the possibility of high-voltage spike due to an alternator "load dump" into the master bus when the battery is diconnected via the master relay.
   In my circuit, the alternator output is always connected to the battery (via a fuse).

After flying it for two years, I can say that all goals were met. I like the way the system works in practice and will probably do something similar on my next build.

Here's the schematic:
http://www.cameron.com/~kcameron/rv-4/Wiring%20Lists%20and%20Schematics/PowerDist.pdf

The power distribution panel fits between the rails behind the firewall along with the battery. Here's a picture:
http://www.cameron.com/~kcameron/rv-4/Wiring Lists and Schematics/PowerDist_view.jpg

With cover and battery:
http://www.cameron.com/~kcameron/rv-4/Pics/2008-12-07--17.43.33/00008.jpg

Here's the main switch panel. The "Off"/"Start"/"Master" switch is on the lower left. Below that is the hinged main fuse panel. Unfortunately, I don't think I ever took a picture of it while open:
http://www.cameron.com/~kcameron/rv-4/Pics/2008-12-07--17.43.33/00013.jpg

Since I know someone will ask:
The schematic entry tool I used is gschem which is part of the free gEDA CAD package. Feel free to grab my schematic and symbols. My avionics wire-lister input and output are also available.

 

[apkp777]

Cool. Looks pretty good.

Question. Why did you choose a 70a fuse with a 60a alternator? And the next obvious question, do you therefore have an unprotected feed line?

 

[kcameron]

Cool. Looks pretty good.
Question. Why did you choose a 70a fuse with a 60a alternator? And the next obvious question, do you therefore have an unprotected feed line?

The fuse is on the distribution panel and protects the wire between it and the alternator from shorts; not from the alternator producing too much current. I chose 70A because I wanted to ensure that it wouldn't blow unless there's a short. Actually I've seen 70A plus from the alternator. I had a bad crimp in the alternator field which gave an intermittent connection. It would fail in flight and I sometimes didn't notice it until the battery was pretty depleted. When it reconnected (either by itself or when I accidentally touched it while groping around behind the panel) the charge current would be quite high for 15 minutes or so. These fuses are slow-blow so they will tolerate small overloads for quite awhile.

 

[fly3g]

In flight re-start?

Neat idea. If you had to do an in-flight re-start it looks like it shuts down your master, does a back up circuit keep the essential things going?

 

[kcameron]

Neat idea. If you had to do an in-flight re-start it looks like it shuts down your master, does a back up circuit keep the essential things going?

If I got to the point where I had to run the starter motor in-flight, I wouldn't really consider any other electronics to be "essential". Just keep a somewhat nose-down attitude and otherwise fly the plane while trying to relight the engine. It's a moot point for me anyway since I do have ASI and ALT steam gauges and an EFIS with internal battery backup.